It seemed sensible to protect the 'Common' branch from meddling by
inexperienced users by setting the access rights so that 'Everyone' had
read-only access and only specific users had full access.

This achieved the desired effect in preventing users making incorrect
changes, but had the unpleasant side-effect that users' settings, such as
the selected default branch, were not remembered. It took some time to
work out why only some users had their settings remembered!